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Two legislative aides to U.S. Sen. Robert Menendez (D-NJ) have declined to answer questions in front of a federal grand jury probing the senator, citing the speech or debate clause of the U.S. Constitution, the New Jersey Law Journal reported on Friday.

Menendez has been under federal criminal investigation in connection with his ties to Florida eye doctor Salomon Melgen. The senator has admitted he accepted free private plane trips from Melgen, including a 2008 trip to the luxury resort of Casa de Campo in the Dominican Republic. The senator has said he later repaid almost $70,000 for his trips on the doctor’s jet.

The Justice Department is looking into whether, in exchange for the gifts, Menendez improperly lobbied U.S. officials to help Melgen with business matters. Two key issues include whether the senator improperly helped Melgen in Melgen’s efforts to secure a port security deal worth tens of millions of dollars, as well as helping the eye doctor with Medicare regulators looking into whether Melgen had overbilled Medicare.

The New Jersey Law Journal (NJLJ) reports the Third Circuit Court of Appeals has ordered a hearing into whether the two Menendez staffers must testify or can claim legislative privilege under Article 1, Section 6 of the Constitution.

Federal judge Anne Thompson previously had ordered Menendez aides Michael Barnard and Kerri Talbot to testify about the meetings they had with Health and Human Services officials, as well as Custom and Border Protection officials about Melgen, the NJLJ report said.

The report said Barnard declined to answer 50 questions during his initial grand jury appearance. A spokesman for Menendez declined to comment on the report.

The Third Circuit Appeals Court said the congressman has the burden of showing the speech or debate clause applies in a case like this. Prosecutors are looking into whether Menendez’s actions on behalf of Melgen were corrupt and not part of the “manifestly legislative” process, NJLJ said.

The documents cited in the NJLJ report are no longer on the courts electronic database suggesting normally sealed grand jury materials might have been posted on the Court’s website by mistake.

Calls made to clerks for Third Circuit Judges D. Michael Fisher, Kent Fisher and Cheryl Ann Krause seeking access to the documents were not returned.

Investigators have said they are also looking into the senator’s relationship with two fugitive Ecuadorean bankers living in Florida. Brothers William and Roberto Isaias have been sentenced in absentia in Ecuador for looting a bank they once ran there. Isaias family members gave campaign donations to numerous politicians including Senator Menendez. Menendez’s office repeatedly contacted State Department and Homeland Security officials in support of the bankers efforts to stay in the US.

Brothers William and Roberto Isaias have said they are innocent of banking crimes in their home country, and are the victims of a political witchunt by Ecuadorean authorities, and along with the senator, deny any wrongdoing.